The invention relates to a multi-gradation image recording apparatus using a heat-sensitive type recording head. More particularly, the invention relates to a multi-gradation type image recording apparatus in which a number of heat generating elements are arranged one-dimensionally in correspondence to picture elements , and a heat-sensitive type recording head designed so as to drive the heat generating elements with binary video signals is used to record an image in an overlap mode.
In a conventional multi-gradation image recording operation using a heat-sensitive type recording head, energy for exciting the heat generating elements (which are provided respectively for the picture elements) is controlled according to the gradations of the picture elements. Therefore, drive elements, the number of which is equal to the number of heat generating elements driven simultaneously, are so controlled as to supply energy corresponding to the required gradations to the respective heat generating elements.
First, a conventional two-valued (binary) image recording apparatus, which is extensively employed, will be briefly described.
FIG. 1 is a block diagram of a circuit used in the two valued image recording apparatus. A binary video signal VS delivered in a time-series mode is loaded into a shift register 1, where it is converted into parallel signals 7.sub.1, 7.sub.2 . . . and 7.sub.5. These signals 7.sub.1 through 7.sub.5 are applied to AND gates 2.sub.1 through 2.sub.5, respectively. The electrical energy of a power source 6 is applied to the remaining input terminals of the AND gates 2.sub.1 through 2.sub.5 where it is ANDed with the parallel binary video signals 7.sub.1 through 7.sub.5.
The outputs of the AND gates 2.sub.1 through 2.sub.5 are applied to respective heat generating resistance elements 3.sub.1 through 3.sub.5 of a heat generating head. The apparatus has block specifying switches 4 and 5, which are closed one at a time so that the heat generating resistance elements which are specified by the closed block specifying switch, and the parallel signals 7.sub.1 through 7.sub.5, are excited to contribute to the image recording operation.
For instance, when the parallel binary video signals 7.sub.1 through 7.sub.5 represent "0 1 1 0 1" and the block specifying switch 4 is closed, current is applied from the power source 6 to the heat generating resistance elements 3.sub.2, 3.sub.3 and 3.sub.5 to cause the latter to generate heat, while no current is applied to the remaining heat generating resistance elements 3.sub.1 and 3.sub.4 ; i.e. the elements 3.sub.1 and 3.sub.4 generate no heat.
When, at the following time instant, the parallel binary video signals 7.sub.1 through 7.sub.5 are changed to represent "1 0 0 1 1" and the block specifying switch 5 is closed, only the heat generating resistance elements 3.sub.6, 3.sub.9 and 3.sub.10 generate heat.
As a result of the above-described operation, a two-valued image the density of which is represented by "0 1 1 0 1 1 0 0 1 1" (where "0" represents white and "1" represents "black", for instance) is recorded on a heat-sensitive sheet.
In order to carry out a multi-gradation image reproduction with the above-described conventional apparatus, it is necessary to both employ multi-level signals whose levels correspond to more than three gradations, as in the aforementioned parallel video signals 7.sub.1 through 7.sub.5, and to identify which gradation each signal represents.
In addition, according to the identification results, either the amount of electrical energy supplied to the AND gates 2.sub.1 through 2.sub.5 from the power source 6 or the timing of opening the AND gates must be controlled.
Therefore, the conventional apparatus is disadvantageous in that it is intricate in construction and low in reliability. In addition, conventional apparatus suffers from the drawbacks that, since the energy applied to the heat generating elements changes with the gradations of a picture to be recorded, as the density of a picture element increases, the size, of the picture element also increases, such that the resolution of the recorded image is decreased, (that is, the recorded image is low in quality).